the life of a material particle is its adventure amid a track of
event-particles strung out as a continuous series or path in the
four-dimensional space-time manifold. These event-particles are the
various situations of the material particle. We usually express this
fact by adopting our natural space-time system and by talking of the
path in space of the material particle as it exists at successive
instants of time.

We have to ask ourselves what are the laws of nature which lead the
material particle to adopt just this path among event-particles and no
other. Think of the path as a whole. What characteristic has that path
got which would not be shared by any other slightly varied path? We are
asking for more than a law of gravity. We want laws of motion and a
general idea of the way to formulate the effects of physical forces.

In order to answer our question we put the idea of the attracting masses
in the background and concentrate attention on the field of activity of
the events in the neighbourhood of the path. In so doing we are acting
in conformity with the whole trend of scientific thought during the last
hundred years, which has more and more concentrated attention on the
field of force as the immediate agent in directing motion, to the
exclusion of the consideration of the immediate mutual influence between
two distant bodies. We have got to find the way of expressing the field
of activity of events in the neighbourhood of some definite
event-particle E of the four-dimensional manifold. I bring in a
fundamental physical idea which I call the 'impetus' to express this
physical field. The event-particle E is related to any neighbouring
event-particle P by an element of impetus. The assemblage of all the
elements of impetus relating E to the assemblage of event-particles in
the neighbourhood of E expresses the character of the field of
activity in the neighbourhood of E. Where I differ from Einstein is
that he conceives this quantity which I call the impetus as merely
expressing the characters of the space and time to be adopted and thus
ends by talking of the gravitational field expressing a curvature in the
space-time manifold. I cannot attach any clear conception to his
interpretation of space and time. My formulae differ slightly from his,
though they agree in those instances where his results have been
verified. I need hardly say that in this particular of the formulation
of the law of gravitation I have d